(CNN) — The odds of surviving a heart attack are better than ever in the United States thanks to improvements in therapies.

But that means about 2.5 million Americans live with heart failure that blocks blood vessels and reduces blood supply to the body.

A new study provides an early indication that cell therapy using cells from bone marrow could one day help treat heart failure.

Researchers gave 60 patients with heart failure an injection of the therapy in the heart and compared their rates of death and heart problems to those of 66 similar patients who received a placebo injection of saltwater in the heart.

The researchers found that, during the year following the treatment, the patients who received the cell therapy had a 37% lower rate of death and hospitalization for heart failure-related problems, such as fluid buildup in the body or shortness of breath, compared with the placebo group. In the cell therapy group, 3.4% of the patients died and 51.7% were hospitalized for heart problems, whereas 13.7% of the placebo group died and 82.4% were hospitalized.

“This would be considered a huge success because this much of a reduction has not been shown with any other (cell) therapy,” said Dr. Amit N. Patel, director of the clinical regenerative medicine program in the University of Utah Department of Surgery.

The therapy is working probably because it either “slows down or reverses the rate of progression of disease,” said Patel, who led the clinical study, which was published on Monday in the journal The Lancet and presented at the annual scientific meeting of the American College of Cardiology. Rather than increasing the number of muscle cells or blood vessels in the heart, the therapy is probably making the muscle cells work better, he added.

The company that developed the cell therapy technology, called Vericel Corp., funded and was involved in the study, although Patel said he had the final say about what was included in the article.

Here’s how the therapy, which is called Ixmyelocel-T, is carried out: The researchers remove about three tablespoons of bone marrow from the hip bone while the patient is lightly sedated. The cells in the bone marrow then grow in an instrument called a bioreactor for two weeks, producing a “soup” of cells containing certain types of stem cells and immune cells that can help remodel tissue and reduce inflammation, Patel said. Finally the researchers use special catheters to identify the weakest parts of the heart and inject the soup into these areas.

In the year after the injection, 20.3% of the patients in the cell therapy group experienced an adverse event such as infection or stroke, compared with 41.8% of the placebo group. “It was surprising that the (placebo) patients did significantly worse,” Patel said. This could have been because they underwent the same invasive procedures as the treatment group, but did not receive the same potentially beneficial cell therapy, which could have anti-inflammatory effects that decreased adverse events, he said.

“The study is very exciting because for the first time it showed a physical impact on clinical events, and in this case that was mortality and rehospitalization for heart failure,” said Dr. Thomas J. Povsic, an interventional cardiologist and associate professor of the Duke Clinical Research Institute. Povsic was not involved in the study, but wrote an editorial about the study for the same issue of The Lancet.

Nevertheless, this therapy will need to be tested on more patients, Povsic said. “Although the study in The Lancet is very encouraging, it’s still a relatively small study by cardiovascular standards. In heart disease we typically study hundreds to thousands of patients,” he said.

The researchers and Vericel are hoping to start a larger phase 3 clinical trial of Ixmyelocel-T that includes more heart failure patients. The new study was a phase 2 trial, and these trials generally focus on establishing the effectiveness and safety of a new therapy.

There are currently several phase 2 and 3 trials underway testing cell therapies for the treatment of various types of heart disease. So far, studies have either not addressed the therapy’s effect on clinical outcomes, or failed to observe an improvement in heart disease.

Earlier cell therapies used all the cells in the bone marrow instead of selecting for certain types of cells, and thus might have been diluting their beneficial effects, Povsic said. “We are moving more and more away from first generation cell therapy,” said Povsic, who is involved in two ongoing trials looking at the effect of selected cells from bone marrow in patients with heart failure or angina, a type of heart disease that reduces blood supply to the heart.

No cell therapies have been approved worldwide for heart failure patients. There are also no approved cell therapies for other types of heart disease in the United States, but several therapies are available in Asia, including a stem cell-based treatment for people who have had a heart attack.

The therapy in the new study would be appropriate for patients who have heart failure and are getting worse, despite taking medicines such as beta blockers and ACE inhibitors, Patel said. Nearly all the patients in the study were on one of these drugs. However, the therapy would not be appropriate for patients whose heart failure is so bad that they are often hospitalized for heart complications, Patel said. These patients would be candidates for a heart transplant or left ventricular assist device.

“There’s a growing population of (heart failure) patients that are on medicines and who continue to have significant symptoms,” Povsic said.